Numerous companies now promote pre-sealing stone products at the time of fabrication. This may seem appealing to an uninformed buyer but it is more of a marketing strategy than a real consumer benefit. Stone sealing is the application of a surface treatment to products constructed of natural stone to retard staining and corrosion.
There are two types of sealers: a penetrating or topical one. A topical sealer, which is a protective coating that covers the stone's exterior, blocks any substances from getting in.
Penetrating sealers penetrate the stone and act as a barrier against staining, preventing many substances from occupying the same space. A penetrating sealer has a greater resistance to wear and tear than a topical sealer.
Before applying the stone, it must be completely dried. This is difficult because of the water needed for cutting and fabricating the stone. The stone's benefits will not be protected if the sealer has been applied while it is still damp.
Sealers are able to occupy "space" in stone that would allow other substances to absorb it. The sealer will have a difficult time "bullying" water past stone that isn't dry. The sealer won't have taken up space from the water molecules when the stone is dry so it will not be sealed properly.
However, it's safer and more cost-effective to apply the sealer once the stone has been installed. Manufacturers that encourage pre-sealing stone at the factory use cheap products that need to be sealed again soon after installation.